In recent years, emission of carbon dioxide, which causes environmental destruction such as air pollution and global warming, has become a problem. There are also concerns about a risk of gas leaks and explosions accompanied by aging of infrastructure. Under such circumstances, an importance of sensor(s) for detecting gas and gas detection system that pose a danger such as environmental load gas or explosion causing environmental destruction is increasing. In particular, in recent years, consumption of natural gas, which emits less carbon dioxide than coal and petroleum, has increased. With the increase of consumption of natural gas, the detection of gas leakage in a natural gas distribution network is regarded as important.
Here, the main component of natural gas is methane molecule (CH4). Semiconductor type sensor(s) are often used to detect methane molecules. Semiconductor type sensor can detect, as a gas concentration value, a change in resistance that occurs when the metal oxide semiconductor contacts gas. Furthermore, the semiconductor sensor(s) is not only for flammable gas, there is an advantage that it is possible to detect the various gases such as toxic gas.
However, since the power supply to a sensor part is needed for use of the semiconductor type sensor, the sensor part (sensor part which detects gas) is required to have an explosion-proof structure. Also, for reasons such as the sensor part has movable positions, administrative tasks such as requirement of periodic inspection and calibration work or taking time to gas detection because of measuring by contacting the gas, which are counted as problems.
Thus, for a safety use of semiconductor type sensor, there is much of problems and requirements, so that alternative means of semiconductor type sensor have been studied. For example, there are sensors using a phenomenon in which a light transmitting through gas is attenuated by infrared absorption, and gas detection systems using such sensors. In these gas detection systems, concentration by each kind of gas is estimated from absorption wavelength of light corresponding to the kind of gas and attenuation amount of the transmitted light.
Patent Literature 1 discloses a method of detecting gas concentrations at multiple locations. Patent Literature 1 discloses a multipoint gas concentration measurement method in which control method is simple and amount of optical fibers used may be small. Patent Literature 2 discloses a gas detection apparatus that detects gas at a plurality of locations. Non-Patent Literature 1 discloses an optical fiber type gas detection system.
In addition, Patent Literature 3 discloses a gas measurement apparatus, comprising a light source part and a light receiving part which are embedded in a same casing, and the light path is bent by 90 degrees. Patent Literature 4 discloses a gas analyzer, and describes that a partial pressure value is determined from a fluctuation amount of measurement light in gas to be measured. Patent Literature 5 discloses a gas analyzer and a technique to improve accuracy in detecting gas concentration using laser light.